Methods for transferring points of interest between images with non-parallel viewing directions

ABSTRACT

A method for identifying within a working image of a scene a point of interest designated within a designation image of a scene, the designation image being taken along a first viewing direction and the working image being taken along a second viewing direction, the second viewing direction being significantly non-parallel to the first viewing direction, the method comprising the steps of: obtaining a designation image of the scene; obtaining a working image of the scene; correlating said designation image and said working image with each other, directly or by correlating each of said designation image and said working image with a common reference image, so as to derive an interrelation between said designation image and said working image; and employing said interrelation between said designation image and said working image to derive a location within said working image of a point of interest designated within said designation image, characterized in that the method further comprises: obtaining a secondary designation image taken along a viewing direction displaced from, but similar to, the first viewing direction; and co-processing said designation image and said secondary designation image to derive range information relating to a plurality of pixels in said designation image, thereby defining a partial relative three-dimensional model of a portion of the scene relative to the viewing direction of said designation image, wherein said partial relative three-dimensional model is used in said step of correlating.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to the field of image processing and, inparticular, it concerns methods for locating in one perspective view apoint of interest designated in another perspective view where theviewing directions of the two views are significantly non-parallel.

A common problem in the field of multi-platform field operations iscommunication of a point of interest between different platforms.Specifically, it is often desired to designate a particular location(e.g., pixel) within an image of a scene viewed from a first directionand then to determine the corresponding location (pixel) of the samepoint of interest in an image of the scene as viewed from a differentdirection. When the viewing directions are near-parallel, or where thescene is generally flat so that few features are obscured, this task canbe achieved by well developed image processing techniques for imagecorrelation which identify pairs of corresponding features in the twoimages and determine a transformation between the images which mapspixel-to-pixel between the images. For scenes with pronounced verticaltopology, such as urban terrain, and where viewing directions aresignificantly non-parallel, the problem rapidly becomes much moredifficult, if not insoluble.

The source of the aforementioned difficulty is intuitively understood ifone considers two aerial cameras viewing a group of buildings from theWest and from the East, respectively. Clearly, all features on thevertical faces of the buildings are not shared between the two imagesand cannot form a basis for conventional image correlation. Furthermore,if the buildings are densely positioned and the viewing angles shallow,much of the ground area between the buildings may be obscured in one orboth images. As a result, the vast majority of the features in eachimage may not have corresponding features in the other image, leavingrelatively few features, typically including parts of the buildingroofs, in common. As the proportion of corresponding features in the twoimages decreases, the reliability and accuracy of conventional imagecorrelation techniques rapidly declines.

An alternative approach to transferring a point of interest between twoperspective views is by registration of each image to a geographicalimage database. Each image is separately correlated to the correspondingsubregion of an image retrieved from the geographical image database,and hence each pixel of each image is associated with a knowngeographical coordinate. The geographic coordinates then provide acommon language to allow transfer of points of interest from one imageto the other.

The use of a geographical reference image (“orthophoto” or satelliteimage) has certain advantages. Firstly, the use of an overhead viewtends to limit the maximum angular discrepancy between viewingdirections of the images to be correlated. Secondly, the digital terrainmap (DTM) associated with the geographical database provides additionalinformation which can be used to facilitate the correlation processing.Nevertheless, this approach also encounters major problems when dealingwith urban terrain where the DTM typically lacks sufficient resolutionand/or may not be sufficiently up-to-date to define building structuresand the aforementioned problems of insufficient correspondence betweenfeatures tend to occur.

For the above reasons, the generally accepted approach to fieldoperations in urban terrain is that a full three-dimensional model ofthe external surfaces of the building structures should be determined.This may be achieved using structure-from-motion (“SFM”) techniques inwhich features are tracked through successive frames of a video toderive locations of the features in three-dimensions, and the trackedfeatures are then associated to identify surfaces of the structures.This model can then be used to provide additional information for anygiven viewing direction, thereby facilitating registration of eachsubsequent perspective view with the model and the transfer of points ofinterest to or from each image.

Although the three-dimensional model approach is highly effective, it isnot always feasible in practical operations. Specifically, constructionof a three-dimensional model requires acquisition of images from alldirections around the buildings in question as well as computationallyintensive processing. Limitations of time and/or accessibility inhostile territory may preclude this approach.

A further technical hurdle presented by the three-dimensional modelapproach is alignment of the model with the real-world geographiccoordinate system. The geometrical form of the three-dimensional model,typically represented as a precise local elevation map, is afundamentally different and incompatible form of data from the imagedata of an orthophoto or satellite image tied to the geographicalcoordinate system. In many cases, this alignment can only be performedreliably as a laborious manual procedure, rendering the techniqueimpractical for automated or real-time operations.

There is therefore a need for methods for locating in one perspectiveview a point of interest designated in another perspective view wherethe viewing directions of the two views are significantly non-parallel,and particularly in urban terrain where full three-dimensional modeldata is unavailable.

SUMMARY OF THE INVENTION

The present invention is a method for locating in one perspective view apoint of interest designated in another perspective view where theviewing directions of the two views are significantly non-parallel.

According to the teachings of the present invention there is provided, amethod for identifying within a working image of a scene a point ofinterest designated within a designation image of a scene, thedesignation image being taken along a first viewing direction and theworking image being taken along a second viewing direction, the secondviewing direction being significantly non-parallel to the first viewingdirection, the method comprising the steps of: (a) obtaining adesignation image of the scene; (b) obtaining a working image of thescene; (c) correlating the designation image and the working image witheach other, directly or by correlating each of the designation image andthe working image with a common reference image, so as to derive aninterrelation between the designation image and the working image; and(d) employing the interrelation between the designation image and theworking image to derive a location within the working image of a pointof interest designated within the designation image, characterized inthat the method further comprises: (e) obtaining a secondary designationimage taken along a viewing direction displaced from, but similar to,the first viewing direction; and (f) co-processing the designation imageand the secondary designation image to derive range information relatingto a plurality of pixels in the designation image, thereby defining apartial relative three-dimensional model of a portion of the scenerelative to the viewing direction of the designation image, wherein thepartial relative three-dimensional model is used in the step ofcorrelating.

According to a further feature of the present invention, the step ofcorrelating includes employing the range information to generate asimulated image generated by warping the designation image toapproximate a view taken along the second viewing direction andcorrelating the working image with the simulated image.

According to a further feature of the present invention, the step ofcorrelating includes correlating each of the designation image and theworking image with a reference image taken along a reference viewingdirection, and wherein the partial relative three-dimensional model isused to derive a local relative depth map for at least a subregion ofthe reference image.

According to a further feature of the present invention, the referenceviewing direction is intermediate to the first and second viewingdirections.

According to a further feature of the present invention, the referenceimage is a geographically anchored reference image, and wherein thelocal relative depth map is a local relative digital elevation map forat least a subregion of the geographically anchored reference image.

According to a further feature of the present invention, the designationimage and the working image are derived from different imaging sensors.

There is also provided according to the teachings of the presentinvention, a method for identifying within a working image of a scene apoint of interest designated within a designation image of a scene, thedesignation image being taken along a first viewing direction and theworking image being taken along a second viewing direction, the secondviewing direction being significantly non-parallel to the first viewingdirection, the method comprising the steps of: (a) obtaining adesignation image of the scene taken along the first viewing direction;(b) obtaining a secondary designation image taken along a viewingdirection displaced from, but similar to, the first viewing direction;(c) co-processing the designation image and the secondary designationimage to derive range information relating to a plurality of pixels inthe designation image, thereby defining a partial relativethree-dimensional model of a portion of the scene relative to theviewing direction of the designation image; (d) obtaining a workingimage of the scene taken along the second viewing direction; (e)employing the range information to generate a simulated image generatedby warping the designation image to approximate a view taken along thesecond viewing direction; (f) correlating the working image with thesimulated image so as to derive an interrelation between the designationimage and the working image; and (g) employing the interrelation betweenthe designation image and the working image to derive a location withinthe working image of a point of interest designated within thedesignation image.

There is also provided according to the teachings of the presentinvention, a method for identifying within a working image of a scene apoint of interest designated within a designation image of a scene, thedesignation image being taken along a first viewing direction and theworking image being taken along a second viewing direction, the secondviewing direction being significantly non-parallel to the first viewingdirection, the method comprising the steps of (a) obtaining adesignation image of the scene taken along the first viewing direction;(b) obtaining a secondary designation image taken along a viewingdirection displaced from, but similar to, the first viewing direction;(c) co-processing the designation image and the secondary designationimage to derive range information relating to a plurality of pixels inthe designation image; (d) correlating the designation image with areference image taken along a reference viewing direction so as toderive an interrelation between the designation image and the referenceimage, the range information being used to derive a local relative depthmap for at least a subregion of the reference image; (e) correlating theworking image with the reference image so as to derive an interrelationbetween the reference image and the working image, the correlating beingperformed using the local relative depth map; and (f) employing theinterrelations between the designation image and the reference image andbetween the working image and the reference image to derive a locationwithin the working image of a point of interest designated within thedesignation image.

According to a further feature of the present invention, the referenceviewing direction is intermediate to the first and second viewingdirections.

According to a further feature of the present invention, the referenceimage is a geographically anchored reference image, and wherein thelocal relative depth map is a local relative digital elevation map forat least a subregion of the geographically anchored reference image.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a schematic representation of a scenario in which the presentinvention will be illustrated wherein two imaging sensors acquire imagesof an urban scene from different viewing directions;

FIG. 2 is a flow chart illustrating a first implementation of a methodfor transferring points of interest between images taken from differentviewing directions according to the teachings of the present invention;and

FIG. 3 is a flow chart illustrating a second implementation of a methodfor transferring points of interest between images taken from differentviewing directions according to the teachings of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a method for transferring points of interestbetween images taken from different viewing directions, andcorresponding systems for implementing such a method.

The principles and operation of systems and methods according to thepresent invention may be better understood with reference to thedrawings and the accompanying description.

Referring now to the drawings, FIG. 1 illustrates the scenario in whichthe method of the present invention is typically used. Specifically,FIG. 1 illustrates a situation where a first imaging sensor, designated“Sensor 1”, acquires an image, designated “Image 1”, of a scene ofinterest, the image being acquired at a time T₁ and along a firstviewing direction D₁. An imaging sensor “Sensor 2”, which may be anothersensor or may be a later (or earlier) use of the same physical sensor asSensor 1, acquires an image, designated “Image 3”, of an overlappingscene at a time T₂ and along a different viewing direction D₂. Thepresent invention is primarily applicable to situations in which viewingdirections D₁ and D₂ are significantly non-parallel, i.e., at least 20degrees apart. Although not limited to such cases, the present inventionis of particular significance in situations where the viewing directionsare highly non-parallel, such as more than 60 degrees apart, and oftenas much as 90 degrees apart or more. In such cases, and particularlywhen viewing scenes with significant vertical topography such asbuildings, simple image correlation between Image 1 and Image 3 istypically not possible, or at least not reliable. Furthermore, thepresent invention primarily addresses cases where no fullthree-dimensional model of the scene is available, either becauseinsufficient data is available to construct a full model or simplybecause no such model has been calculated.

In general terms, the method of the present invention includes obtaininga second image of the scene (“Image 2”) which is taken along a viewingdirection D₁ displaced from, but similar to, first viewing direction D₁.For this purpose, “similar viewing directions” are taken to be viewingdirections sufficiently close in angle that conventional imagecorrelation can typically be performed reliably by standard techniques,and in this context, typically refers to directions differing by lessthan about 30 degrees. Preferably, the change in viewing direction iskept much smaller than this, and the images are sampled in quicksuccession (i.e., at times T₁ and T₁+ΔT) by the same imaging sensor inorder to maximize the similarity between the images and make correlationof the images as reliable as possible. Optionally, more than two imagesmay be used, for example, a relatively short video sequence. Even insuch cases, the angular spacing of the furthest-spaced images used iskept sufficiently small that reliable pixel-to-pixel registration can beachieved over a majority of the overlapping region of the images. Onceacquired, Image 1 and Image 2 are co-processed to derive rangeinformation relating to a plurality of pixels in Image 1, therebydefining a partial relative three-dimensional model of a portion of thescene relative to the viewing directions. This partial relativethree-dimensional model, or the corresponding depth data, is then usedto facilitate correlating Image 1 and Image 3, either directly or bycorrelating each with a common reference image, so as to derive aninterrelation between Image 1 and Image 3. This interrelation is thenemployed to derive a location within one of the images corresponding toa point of interest designated within the other of the images.

At this point, it will already be clear that the method of the presentinvention is highly advantageous, providing additional depth informationfor one of the viewing directions to facilitate accurate and reliablecorrelation to images from widely differing viewing angles, even wheninsufficient information or resources are available for derivation of afull three-dimensional model. Two practical implementations of how theadditional information may be used will be presented below withreference to FIGS. 2 and 3, respectively.

Before proceeding with the details of practical implementations of theabove principles, it will be helpful to define certain terminology asused herein the description and claims. Firstly, Image 1 and Image 2 arereferred to herein as “designation images” while Image 3 is referred toas a “working image”. This terminology is used merely as a convenience,and is chosen according to the most common usage of the system andmethod of the present invention according to which a point of interestis designated in one of Images 1 and 2, and needs to be identified in alater real-time “working image” (Image 3). It will be appreciatedhowever that, once a transformation has been established between Image 1and Image 3, points of interest may readily be transferred in eitherdirection. Thus, an application where a point is designated in the“working image” and the corresponding point is identified in the“designation image” also falls within the scope of the presentinvention.

The term “point of interest” is used to refer to any location or regiondesignated in one image which is to be identified in another image. Inmany cases, the “point” will be identified as a pixel, or small group ofpixels, in the image in which it is designated. In certain cases, the“point of interest” may be a larger area determined by a boundary line,in which case each point used to specify the boundary may be regarded asa “point of interest” in its own right. Furthermore, in certain cases,the point of interest may be specified with sub-pixel accuracy relativeto the designation images themselves, for example, if a plurality ofdesignation images are used to derive a higher resolution (or“super-resolution”) image. The resolution with which the point ofinterest is specified may differ between the working image and thedesignation image.

The term “correlating” is used herein in the description and claims torefer to any processing of two or more images which results indetermining an interrelation between the images. The term“interrelation” is used herein to refer to a mapping, transformation orother relation which defines correspondence of at least a subset ofpixels between the images in question.

The term “warping” is used herein to refer to any image processingtechnique which renders an image sampled along a first viewing directionso as to approximate how the image would appear if viewed along a secondviewing direction. Although the term “warping” thus defined wouldinclude affine transformations and other planar transformations, the“warping” referred to in the present invention typically employs depthinformation associated with the image, as will be detailed below. As aresult, in certain cases, the resulting warped image may be only apartial image, with certain image data from the starting image discardeddue to being obscured in the warped view, and other data expected to bevisible from the new viewing direction unavailable in the startingimage.

The phrase “viewing direction” is used herein to refer to a directioncorresponding to the optical axis of an image sensor at the time animage is sampled. The angle between two viewing directions is definedintuitively, and may be defined algebraically as the arccosine of thescalar product of unit vectors along the two viewing directions.

A third viewing direction is considered to be “intermediate” to twoother viewing directions if the angle formed between the third viewingdirection and each of the other viewing directions is less than theangle measured directly between the other two viewing directions.

Reference is made to range information generating a “partial relativethree-dimensional model of a portion of the scene.” It should be notedthat the partial model of the present invention is effectively athree-dimensional impression of the scene taken from a single viewingdirection, in clear contrast to the panoramic models required by theaforementioned SFM techniques. Specifically, according to theconventional SFM approach, a partial three-dimensional model constructedbased on views taken only from the east side of a group of buildings(and hence lacking all information regarding the westward facingsurfaces of the buildings) would be assumed to be useless forcorrelation with another view taken from the west side of the buildings.The techniques of the present invention address this problem ofincomplete information, or insufficient resources to assemble theinformation, as detailed further below.

The phrase “local relative depth map” is used to refer to a collectionof relative depth data associated with locations within a twodimensional image. As will become clear from the description below, thedepth map in question need not correspond to range data from anyparticular image sensor. “Depth” in this context refers to distance in adirection generally parallel with the effective viewing direction forthe image in question. In the case of a high-altitude aerial photo,satellite photo or an “orthophoto” (a processed image with aneffectively vertical viewing direction at all points), this becomes a“local relative digital elevation map.” The depth map is described as“local” in the sense that it may cover a limited subregion or “locale”of a much more extensive image, for example, from a geographical imagedatabase. The depth map is described as “relative” in the sense that itis internally consistent, but need not be determined in absoluteposition. The density of the depth map may be the same as, lesser than,or greater than, the pixel density of the corresponding image.

Referring now again to FIG. 1, it is generally assumed as a startingpoint for the present invention that approximate data is available forthe positions and viewing directions of the Image Sensors. Suchinformation may be derived from any one, or combination, of a wide rangeof sensor systems commonly in use, including but not limited to: GPS;inertial sensor systems; optical tracking; and image correlation togeographic reference databases. The tracking technologies need not bethe same for both image sensors.

The image sensor used for deriving the designation images may bedistinct from the image sensor for deriving the working image, and mayemploy different imaging technology and/or be sensitive to differentwavelengths of visible or invisible light. Thus, for example, thedesignation images may be color images while the working image is a grayimage, or vice versa, or one or both of the image sensors may operate inone or more infrared wavelength band. Despite all of the above options,an implementation wherein the same image sensor is used at differenttimes to sense the designation images and the working image also fallswithin the scope of the present invention.

The present invention is applicable to any situation where a scene isviewed by imaging sensors from significantly non-parallel viewingdirections, independent of the platform supporting the imaging sensors.In certain cases, one or more of the sensors may be mounted on aland-based platform, for example, a fixed vantage point or a vehiclelocated with a suitable range of view. More typically, the invention isapplied to images retrieved from sensors mounted on flying or otherwiseairborne platforms. Examples of platforms include, but are not limitedto: manned aircraft; unmanned aerial vehicles (“UAVs”) of all types andsizes, lighter-than-air floating platforms, ballistic projectiles andguided weapons.

Turning now to FIG. 2, there is illustrated a first embodiment of amethod according to the teachings of the present invention foridentifying within a working image of a scene a point of interestdesignated within a designation image of a scene. In this embodiment,correlation is performed directly between the designation images and theworking image, assisted by the aforementioned range information.

Specifically, steps 100 and 102 correspond to sampling of designationimages, Image 1 and Image 2, at times T1 and (T1+DT), respectively. Asstated before, the two images are preferably acquired by the sameimaging sensor with differing but similar viewing directions, andpreferably in quick succession, i.e., with a small DT, in order toensure similar lighting conditions and minimize the effect of any movingobjects. In a typical case, the images may be sampled from a moving UAVat times separated by less than a second.

Then, at step 104, the two designation images, together with thecorresponding sensor data, i.e., image sensor position and viewingdirection at the time of sampling the image, are co-processed by imageregistration techniques to derive range information relating to at leasta subset of the pixels in Image 1. (Parenthetically, it will be notedthat Image 1 and Image 2 are interchangeable for this purpose, such thatthe later sampled image may be considered Image 1 and DT may be definedas negative.) The result is referred to as a “dense range map” to theextent that it preferably provides range data at locations in the imagesufficiently densely packed to form a reliable indication of majorsurfaces of buildings within the field of view of the designationimages. In most preferred cases, the range data is derived with aspacing in the same order of magnitude as the pixel resolution of theimage, and in one particularly preferred implementation, for each pixelof Image 1.

Independent of the above process, Image Sensor 2 samples Image 3 at sometime T2 (step 106), and corresponding position and viewing directiondata are determined. The task to be performed by the system and methodof the present invention is to allow identification of correspondingpoints of interest between Image 1 and Image 3.

To this end, according to this embodiment of the present invention, step108 employs the range information from step 104 and the sensor data fromstep 106 to generate a simulated image generated by warping designationimage, Image 1, to approximate a view taken along the viewing directionof Image 3. Then, at step 110, this simulated image is correlated Image3. The parameters of this correlation combined with the parameters ofthe warping together define an interrelation (a global transformation ora set of pixel-to-pixel correspondences) between Image 1 and Image 3.This interrelation then allows identification in either image of a pointof interest designated in the other image.

Turning now to FIG. 3, there is shown an alternative embodiment of themethod of the present invention. This embodiment is generally similar tothe embodiment of FIG. 2, but employs a reference image taken along areference viewing direction.

Thus, as before, designation images, Image 1 and Image 2, are sampled attimes T1 and (T1+DT), corresponding to steps 200 and 202 respectively.At step 204, the two designation images, together with the accompanyingdata (to be described more fully below), are co-processed by imageregistration techniques to derive a dense range map relating to at leasta subset of the pixels in Image 1. At least one of the designationimages is registered with the reference image (obtained at 205) togenerate a transformation TR1 (step 206) which interrelates Image 1 withthe reference image, and the transformation together with the denserange map are used at step 208 to derive a local relative depth map forat least a subregion of the reference image. Most preferably,registration step 206 and a parallel process 206′ for Image 2 areperformed prior to the co-processing of step 204 and the additional datamade available through derivation of the transformations TR1 and TR2 areused to further refine the accuracy of the co-processing. Additionally,or alternatively, the local relative depth map derived by casting thepartial relative three-dimensional model of the scene along thereference viewing direction is used to enhance or render more robust thecorrelation of Image 1 with the reference image. It will be noted thattransformation TR1, which again may be a global transformation or apixel-by-pixel mapping, already provides sufficient information to mappoints of interest in both directions between Image 1 and the referenceimage, as indicated by step 210.

At step 212, Image Sensor 2 samples Image 3 at some time T2, andcorresponding position and viewing direction data are determined. Theimage and sensor data are then used, together with the local relativedepth map, to correlate Image 3 to the reference image to generatetransformation TR3 (step 214). The availability of the local relativedepth map again renders this correlation more reliable and precise thanwould otherwise be possible. Transformation TR3 then allows mapping ofpoints of interest in both directions between Image 3 and the referenceimage, as indicated by step 216. Steps 210 and 216, taken together, thusprovide for transfer of points of interest between Image 1 and Image 3.

While the above described method may be applied with substantially anyreference image and reference viewing direction, it is clearlyadvantageous to choose a reference viewing direction which isintermediate to the first and second viewing directions, therebyrendering the angular discrepancy between images to be correlated. Forthis reason, in a wide range of applications, a reference image with anear-vertical viewing direction is advantageous as an intermediate anglereference image for correlating between images taken from opposingdirections of a region of interest.

Clearly, a likely choice for a source of reference images is a databaseof geographically anchored reference images. The present invention maybe employed in a case where no digital terrain map (DTM) is available,or may employ a DTM where available in deriving the registration of thedesignation image(s) to the reference image. Even in the latter case,however, it should be noted that the local relative depth map derivedfrom the designation images need not be registered with the DIM. Sincethe local relative depth map typically has a much higher resolution thana wide-area DTM, and is up-to-date as of the time of sampling of Images1 and 2, it provides an important tool for achieving more reliable andprecise registration between Image 3 and the reference image.

According to another optional feature of this embodiment, as mentionedin the background section above, the primary choice of features commonto widely spaced viewing directions in a dense urban landscape aretypically associated with roofs of buildings. Accordingly, in certainimplementations, it may be advantageous to apply a filter or mask basedon the local relative depth map to selectively give increased weightduring the registration process to pixels of the reference imageidentified as corresponding to rooftops or other elevated features.

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe scope of the present invention as defined in the appended claims.

1. A method for identifying within a working image of a scene a point ofinterest designated within a designation image of a scene, thedesignation image being taken along a first viewing direction and theworking image being taken along a second viewing direction, the secondviewing direction being significantly non-parallel to the first viewingdirection, the method comprising the steps of: (a) obtaining adesignation image of the scene; (b) obtaining a working image of thescene; (c) correlating said designation image and said working imagewith each other, directly or by correlating each of said designationimage and said working image with a common reference image, so as toderive an interrelation between said designation image and said workingimage; and (d) employing said interrelation between said designationimage and said working image to derive a location within said workingimage of a point of interest designated within said designation image,characterized in that the method further comprises: (e) obtaining asecondary designation image taken along a viewing direction displacedfrom, but similar to, the first viewing direction; and (f) co-processingsaid designation image and said secondary designation image to deriverange information relating to a plurality of pixels in said designationimage, thereby defining a partial relative three-dimensional model of aportion of the scene relative to the viewing direction of saiddesignation image, wherein said partial relative three-dimensional modelis used in said step of correlating.
 2. The method of claim 1, whereinsaid step of correlating includes employing said range information togenerate a simulated image generated by warping said designation imageto approximate a view taken along said second viewing direction andcorrelating said working image with said simulated image.
 3. The methodof claim 1, wherein said step of correlating includes correlating eachof said designation image and said working image with a reference imagetaken along a reference viewing direction, and wherein said partialrelative three-dimensional model is used to derive a local relativedepth map for at least a subregion of said reference image.
 4. Themethod of claim 3, wherein said reference viewing direction isintermediate to said first and second viewing directions.
 5. The methodof claim 3, wherein said reference image is a geographically anchoredreference image, and wherein said local relative depth map is a localrelative digital elevation map for at least a subregion of saidgeographically anchored reference image.
 6. The method of claim 1,wherein said designation image and said working image are derived fromdifferent imaging sensors.
 7. A method for identifying within a workingimage of a scene a point of interest designated within a designationimage of a scene, the designation image being taken along a firstviewing direction and the working image being taken along a secondviewing direction, the second viewing direction being significantlynon-parallel to the first viewing direction, the method comprising thesteps of: (a) obtaining a designation image of the scene taken along thefirst viewing direction; (b) obtaining a secondary designation imagetaken along a viewing direction displaced from, but similar to, thefirst viewing direction; (c) co-processing said designation image andsaid secondary designation image to derive range information relating toa plurality of pixels in said designation image, thereby defining apartial relative three-dimensional model of a portion of the scenerelative to the viewing direction of said designation image; (d)obtaining a working image of the scene taken along the second viewingdirection; (e) employing said range information to generate a simulatedimage generated by warping said designation image to approximate a viewtaken along said second viewing direction; (f) correlating said workingimage with said simulated image so as to derive an interrelation betweensaid designation image and said working image; and (g) employing saidinterrelation between said designation image and said working image toderive a location within said working image of a point of interestdesignated within said designation image.
 8. A method for identifyingwithin a working image of a scene a point of interest designated withina designation image of a scene, the designation image being taken alonga first viewing direction and the working image being taken along asecond viewing direction, the second viewing direction beingsignificantly non-parallel to the first viewing direction, the methodcomprising the steps of: (a) obtaining a designation image of the scenetaken along the first viewing direction; (b) obtaining a secondarydesignation image taken along a viewing direction displaced from, butsimilar to, the first viewing direction; (c) co-processing saiddesignation image and said secondary designation image to derive rangeinformation relating to a plurality of pixels in said designation image;(d) correlating said designation image with a reference image takenalong a reference viewing direction so as to derive an interrelationbetween said designation image and said reference image, said rangeinformation being used to derive a local relative depth map for at leasta subregion of said reference image; (e) correlating said working imagewith said reference image so as to derive an interrelation between saidreference image and said working image, said correlating being performedusing said local relative depth map; and (f) employing saidinterrelations between said designation image and said reference imageand between said working image and said reference image to derive alocation within said working image of a point of interest designatedwithin said designation image.
 9. The method of claim 8, wherein saidreference viewing direction is intermediate to said first and secondviewing directions.
 10. The method of claim 8, wherein said referenceimage is a geographically anchored reference image, and wherein saidlocal relative depth map is a local relative digital elevation map forat least a subregion of said geographically anchored reference image.